The present invention in general relates to a space transfer vehicle ("STV"), and more particularly, relates to a STV having a thruster configuration suitable for maneuvering the vehicle alone or in combination with a payload.
STVs are spacecraft that are launched into orbit in outer space for the purpose of servicing other spacecraft (hereinafter referred to as payloads). In other words, the STV functions as a space tug. The STV are designed to rendezvous, dock and maneuver with and without payloads. This necessitates the ability to execute six degrees of freedom (6 DOF) maneuvers within specific rotational and translational acceleration constraints. This maneuvering is accomplished by the reaction control system, using thruster rocket engines to provide the necessary force. The thruster rockets typically provide a fixed thrust that translates the STV and/or produces rotational torque, based on their size, orientation, and placement with respect to the center of gravity of the vehicle alone or in combination with a payload.
The STV includes a conventional guidance, navigation and control system which has specific operating limits of maximum and minimum translational and rotational acceleration. The lower limit on acceleration is typically 0.01 ft/s.sup.2 for translation and 0.01 deg./s.sup.2 for rotation, while the upper limits on acceleration are 1.0 ft/s.sup.2 for translation and 1.0 deg./s.sup.2 for acceleration. In order to guarantee stability of the STV, maneuvering must be accomplished within these constraints, both with and without a payload attached to the STV.
FIG. 1 is a prior art isometric sketch of the nominal thruster configuration for a STV which is currently the standard configuration in the industry.
In this configuration, six degree of freedom maneuvering of the STV without a payload is provided by incorporating thruster sets positioned at the ends of the vehicle in order to bracket the vehicle center of gravity. In this simplest scenario, translation is accomplished by firing balanced pairs of thrusters, that do not produce any torques or translation other than that desired. Rotation is provided by firing thrusters on opposite sides of the vehicle to provide a pure torque couple. Given the fixed geometry of the vehicle, the thruster size is varied to yield accelerations within the limits of the guidance, navigation and control system. As a practical matter, the minimum acceleration limit is experienced at the beginning of the mission when the STV is fully loaded with propellant. Conversely, the maximum acceleration is experienced when the STV is almost depleted of propellant. In the nominal configuration, a minimum of 24 thrusters is required for 6 DOF maneuvering.
In FIG. 1, as well as the other figures, bold faced arrows are used to represent the thrusters and show the direction of force produced by the thruster. A circle with a dot in its center indicates a thruster having a direction of force produced by the thruster directly out of the page. Thus, each such vector is aligned with an axis called the "thruster axis" or "reaction line of thrust".
In FIG. 1, the three visible faces of the isometric sketch are drawn to include the thruster axes for the prior art nominal configuration. Each of the three faces not visible includes thrusters mounted to produce thruster axes identical to its respective opposite face. Accordingly, a thruster is mounted in each corner of each face to generate a reaction line of thrust perpendicular to the face. Two additional thrusters are mounted in a canted position as shown on both the +Z and -Z faces.
The difficulty with known vehicles used to perform the functions is that the thrusters had no relevancy to the control system or the range of tasks to be carried out. If the thrusters are too big, they will rotate or accelerate the STV more quickly than the control system can accommodate. If a thruster size is chosen which is capable of maneuvering the vehicle by itself within the control system constraints, the thrusters turn out to be too small once a large payload is attached. This nominal thruster configuration of the prior art cannot easily or inexpensively be modified to overcome this inadequacy.
The solution often heretobefore incorporated was to incorporate an additional set of large thruster rockets, in addition to the small set. The smaller set would be used to maneuver the STV up to the docking position and would then be supplemented by the second, larger, set of thrusters to meet the needs of the system once it is docked with a payload. Not only are the thrusters themselves very expensive, but when a second set of thrusters is incorporated, the system becomes substantially more complex. Further, the overall weight and size of the STV is prohibitively increased.
When the STV docks with a payload, the control system must now deal with increased weight and a new center of gravity for the STV payload. For sufficiently large payloads, the new center of gravity of the STV and attached payload moves to a point outside the STV. Accordingly, the center of gravity is no longer enclosed by the reaction control system, and the thrusters produce undesired rotational torques when translation perpendicular to the payload is commanded. This will, in turn, necessitate the firing of rotational thrusters to counterbalance this torque. This effect, along with the increased weight and inertia of the combined vehicles, necessitates a larger thruster size to provide minimum control. However, as discussed above, a larger thruster size will violate the maximum acceleration constraints when the STV is without a payload, and will result in instability.
The result of this conflict is that the nominal space transfer vehicle configuration cannot satisfy the requirements for maneuvering both with and without a payload. Various solutions to this problem have been suggested heretobefore. For example, as discussed above, one solution is to complement the thrusters of the nominal configuration with additional thrusters of varying sizes, making the minimum number of thrusters required for 6 DOF maneuvering 48. A second solution is to attach an independent thruster module to the end of the payload opposite the STV. A third solution would be to replace the thrusters of the nominal configurations with throttleable thrusters.
None of these solutions is, however, without its problems. Incorporating additional thrusters or providing an independent thruster module would require the implementation of both complex and cost prohibitive modifications. Modifying the nominal configuration to include throttleable thrusters would currently increase the thruster cost by at least a magnitude of ten. Further, none of the solutions heretobefore suggested provide the flexibility to meet the control limits of the STV guidance, navigation and control system with and without a payload, incorporating a single thruster size throughout and utilizing a minimum number of thrusters.
Accordingly, it is desired to provide a thruster arrangement for a space transfer vehicle able to control the vehicle through six degree of freedom maneuvering with and without a payload, incorporating a single thruster size throughout and utilizing a minimum number of thrusters.
Generally, in one embodiment, the present invention provides a space transfer vehicle for producing torque about pitch and yaw axes with a first magnitude for maneuvering without a payload and a second magnitude for maneuvering with a payload. The STV has a longitudinal axis coincident with the roll axis, a front end and an aft end. The STV includes a plurality of substantially identical, non-throttleable thrusters, and control means, in the form of a conventional guidance, navigation and control system, for selectively energizing two thrusters simultaneously to produce a resultant torque. The vehicle arrangement includes a plurality of thruster pods equally spaced relative to each other about the roll axis. A plurality of thrusters is mounted in each of the plurality of thruster pods.
Additional objects, advantages, and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.